Solar Orbiter: Status and Schedule Richard Harrison, Rutherford Appleton Laboratory A high-resolution mission to the Sun and inner heliosphere

Solar Orbiter: Status and Schedule

Richard Harrison, Rutherford Appleton Laboratory

A high-resolution mission to the Sun and inner heliosphere

ESA Review of Missions: ‘Cosmic Vision’ - the new ESA Science Programme - released May 27 2002 - Confirms Solar Orbiter as approved mission, in single ‘project’ with BepiColombo

AO 2005(ish), Launch 2011-2012

Solar Orbiter Payload Working Group - Set up by ESA; kick-off meeting 16/17 May 2002 [Remote sensing: Richard Harrison (chair), Udo Schühle, Alan Gabriel, Luca Poletto, Louise Harra]

1. Status Since Last Meeting

2. Mission Concept

Solar Orbiter

SEP & planetary flybys, to achieve 150 day, 0.2 AU perihelion orbit, climbing out of ecliptic.

3. Scientific Goals & Mission Firsts

Mission Firsts

• Explore the uncharted innermost regions of our solar system

• Study the Sun from close-up (45 solar radii or 0.21 AU)

• Fly by the Sun tuned to its rotation and examine the solar surface and the space above from a co-rotating vantage point

• Provide images of the Sun’s polar regions from heliographic latitudes as high as 38°Solar Orbiter

4. Mission Overview

Solar Orbiter

Orbit : solar orbits achieving high heliographic latitudes (up to 38°) with perihelion ~0.2 AU, and co-rotation phases

Launch: 2011-2012 (three launches in Bepi/Orbiter project); windows in principle every ~ 19 months; Soyuz-Fregat from Baikonur assumed Mission duration: cruise phase ~1.9 years (3 orbits); nominal mission ~2.9 years (7 orbits); extended mission ~2.3 years (6 orbits)

Spacecraft: 3-axis stabilised, Sun-pointed (absolute ± 3arcmin, stability ± 0.7 arcsec/15min. SEP technology benefits from BepiColombo; 1296 kg lift-off mass

Payload resources: 130 kg, 127 W, 74.5 kbit/s.

4. Mission Overview

Solar Orbiter

5. Strawman Payload

Solar Orbiter

5. Strawman Payload

Solar Orbiter

6. Aims of This Meeting

Solar Orbiter

To tackle specific technical issues which are fundamental to the design concept of the EUS or which are necessary to demonstrate feasibility. This must be done before the AO!

This includes:

Optical design approach Thermal design/strategy Detector approach

The meeting is a workshop - i.e. We need open discussion to reach decisions!

The EUS Instrument

Requirements

Solar Orbiter

The Need for a UV/EUV Spectrometer

1. UV/EUV spectral range critical for solar plasma diagnostic analysis; provides foundation for exploring the physics of a huge range of phenomena.

2. Current and near-future UV/EUV capability:

Yohkoh None SOHO EUV 2-3” and 0.1 Å & UV 1” and 0.02 Å.TRACE NoneSTEREO (2005) No spectroscopy.Solar-B (2005) EUV 1” & 0.01 Å; on 2” platform;

coronal selection with little TR capability.SDO (2007) Possibly none; EUV/UV spectroscopy - low priority. Solar Probe (??) NoneSolar Orbiter (20011/12) Yes!!!!!

The EUS Instrument

Requirements

Solar Orbiter

Proposed Requirements

Spatial Resolving Element (pixel) 0.5 arcsec 75 km at perihelion

Spectral Resolving Element (pixel) 0.01-0.02 Å/ pixel lower the better

Field of View (minimum) 34 x 34 arcmin2 AR size at perihelion

Exposure time (minimum) <1 s

Maximum Exposure Time Few 100 s cosmic ray limit

Wavelength Bands 170-220 Å580-630 Å> 912 Å

Prime bands f romTenerif e meeting

Pointing To anywhere on Sun and low corona

The Sun’s atmosphere is a truly dynamic, fine-scale environment. Current imaging resolutions (0.5” & few S) are restricting; A consideration of filling factors and basic processes shows that we need to do better. Target: OM improvement in spatial resolution, and 5x better than the best imager capability (75 km on the Sun’s surface, i.e. 0.1” from 1 AU, is 0.5” at 0.2 AU)

The EUS Instrument

Requirements

Solar Orbiter










We need to separate emission lines, but the spectral resolution is driven by velocity, which is a major parameter for this mission; we will have good viewing of the polar outflows for the first time. A value of order 5 km/s would be a reasonable target, i.e. about 0.01 Å/pixel. Use of centroiding could allow some relaxation of this to 0.02 Å/pixel.

The EUS Instrument

Requirements

Solar Orbiter










The FOV is important, especially for a mission in such an eccentric solar orbit. The requirement is for the FOV to cover an active region at the 0.2 AU perihelion, and to cover the full Sun at aphelion. This can be achieved with a FOV of 34’ and upwards (300,000 km square field at 0.2 AU, which is larger than the CDS 200,000 km field; at 1 AU this is 1,500,000 km square.

The EUS Instrument

Requirements

Solar Orbiter










The dynamic nature of the solar atmosphere demands significantly better temporal resolution than currently available. There is a play-off between exposure time and temporal resolution; reasonable counting statistics must be obtained. The actual resolution will depend on the line used and the solar target. The instrument must have flexibility, down to under 1 s.

The EUS Instrument

Requirements

Solar Orbiter










We have to consider a maximum exposure time. Normal operations would require exposures of order 1-50 s. Long exposures will suffer from excessive cosmic ray hits and values in excess of 100 s (e.g. on SOHO) would tend to be swamped by particle hits. Long accumulations can be achieved by summing consecutive images. 100 s is a reasonable upper limit.

The EUS Instrument

Requirements

Solar Orbiter










We require a wavelength range to cover a good temperature range from chromosphere to flare plasmas, with sufficient diagnostic tools. This most likely requires several bands. The bands listed are those favoured at the 2001 Tenerife workshop.

The EUS Instrument

Requirements

Solar Orbiter










We require that the instrument can be pointed to anywhere on the solar disc or low corona. Most likely to be done using the spacecraft - i.e. all pointed instruments work together.

The EUS Instrument

Requirements

Solar Orbiter

Other Factors Which Influence the Design

I nstrument Size Max. Length 2.5 m Due to spacecraf t sizeMass 25 kg target Under 30 kg

Telemetry 20 kbit/ s target Demands large on boardmemory

Power 30 W target

Thermal Environment Varying and high levels ofheat input requiringcaref ul control .

Due to solar proximity andeccentric orbit.

Particle Environment Varying levels of particleevents with some extreme‘storms’. I ncludes solarneutrons.

Cosmic ray backgroundand solar events.

Autonomy Pre-planned sequences indef erred command store.

No contact f or solarpasses

Optical Correction May require active imagestabilisation system.

Spacecraf t stability to bedefi ned.

Under 2.0 m more realistic

Includes ability to cope with latch-up

The EUS Instrument

Web site/documentation

Solar Orbiter

1. Concept document (‘Blue Book’)

2. Technical notes (TN1 - Wavelength selection; TN2 - Orbiter goals; TN3 - Optical design requirements; TN4 - Detector requirements etc…)

3. Contact info., links, Solar Orbiter information, notes/documents...

The EUS Instrument

Concept & Initial Design Strategy

Solar Orbiter

The Original Strawman Design: Ritchey-Chretien feeding spectrometer - 2 reflections to restrict length - retains desired resolution. EFL = 3.7 m. Size: 15cm x 230cm x 55cm.

The EUS Instrument


Solar Orbiter

Secondary mirror can be rotated to ‘raster’ image across slit. Other options include rotation of primary, scan mirror or pointing instrument.

The EUS Instrument


Solar Orbiter

Selection of slits? Slit plane ‘imager’.

The EUS Instrument


Solar Orbiter

Grating baseline was 4800 l/mm but spherical VLS grating option is most likely choice (reduces off-axis aberrations; reduces instrument envelope)

The EUS Instrument


Solar Orbiter

Active Pixel Sensor detector baselined. Better suited to particle environment. Initial design: 9 micron 4kx4k array. Considering: 5 micron.

The EUS Instrument


Solar Orbiter

Initial Thermal concept: Dedicated radiators to primary, secondary and detector, reduced secondary mirror, gold-coating.

Solar ‘constant’ 34,275 W/m2 to 2,142 W/m2 on 149 day cycle (1 AU = 1,371 W/m2)

The EUS Instrument


Solar Orbiter

Two design concepts now under discussion

- Off-axis NI telescope with VLS grating (Martin Caldwell)

- Wolter II GI telescope with VLS grating (Luca Poletto)

We must consider the merits of both options equally

We must keep in mind that any one option may not cater for the requirements - are there any hybrid options?

The EUS Instrument


Solar Orbiter

Martin Caldwell - Off-axis NI design with VLS grating

12cm primary

secondary heat stop

slit

Raster with primary? (minimise heat stop aperture)

The EUS Instrument


Solar Orbiter

- Better thermal situation than on-axis design - but feasibility not yet demonstrated!

- With 5 micron pixels = 1.4m length OK

- Problem: Achieves 0.5” on-axis. Aberrations such that performance off-axis considerably worse than pixel size can we optimise design?

The EUS Instrument


Solar Orbiter

Luca Poletto - Wolter II GI design with NI VLS grating

Configuration C: grazing-incidence telescope and normal-incidence VLS-grating spectrometer (2/3)

Telescope Wolter IIField of view 34 arcmin (|| to the slit,

simultaneous)20 arcmin ( to the slit, acquired by rastering)

Entrance aperture Size 55 mm 55 mmPrimary mirror Paraboloid Size 200 mm 55 mm Incidence angle 74Secondary mirror Hyperboloid Distance from the primary 200 mm Distance from the slit 1550 mm Size 190 mm 40 mm Incidence angle 78Focal length 2310 mm

Mirror for the rastering Plane Distance from the slit 100 mm Size 110 mm 24 mm Incidence angle 82

Slit Size 10 m 23 mm Resolution to the slit 0.9 arcsec

Grating Spherical VLS Central groove density 2400 lines/mm Wavelength 1160-1260 Å (I order)

580-630 Å (II order) Entrance arm 600 mm Exit arm 1200 mm Incidence angle 10 Radius 790 mm Size 15 ( to the grooves) 45 mm Coating SiC

Detector Pixel size 18 m Format 1600 2600 pixel Area 29 ( to the slit) 47 mm Spectral resolving element 62 mÅ (I order)

31 mÅ (II order) Spatial resolving element 0.8 arcsec

The EUS Instrument


Solar Orbiter

- Good thermal capability - much better than NI option.

- So far, optimised to 18 micron pixel (0.8arcsec). Can it be optimsed to 5 micron?

- Overall length = 2.5 m. Too long? Reduced pixel size would help?

- Off-axis aberrations still too great - need optimising.

The EUS Instrument


Solar Orbiter

Key Issues:

- Is 0.5 arcsec critical? TRACE does it now!

- Can the designs cope with the thermal load - especially NI??

- How does each design cope with the thermal variability?

- Will degradation and contamination of optical surfaces, especially in NI, be a critical issue in deciding between NI and GI?

- Is the image stabilisation demand too great for a 0.5 arcsec pixel?

- What about flux?

The EUS Instrument


Flux:

Note 1: CDS area is per grating for NIS. Total CDS effective area consistent with Lang et al. (2002, ISSI Intercal. Workshop)

The EUS Instrument


Flux:

Note 2: S.O./NI aperture circular 12 cm diam. Assume NO multilayer and assume filter, for both S.O. options. Real difference is in telescope, scan mirror & area. ASSUMES SAME DETECTOR FOR BOTH S.O. OPTIONS.

The EUS Instrument


Flux:

Note 3: MCP burn-in in CDS shows selective sensitivity drop to, say, 40-50% in line cores. Effective area = 2.6 x 10-4?

The EUS Instrument


• Relative effective areas CDS = 1.0, SO/NI = 0.4, SO/GI = 0.54

• S.O. figures could improve given an appropriate application of multilayers, more refined figures for detectors etc… Perhaps the removal of the filter?

• There are 13.5 S.O. pixels per CDS pixel. Can we cope with this? Could we live with a CDS with reduced intensities representative of this level? The intensities will NOT be evenly distributed so we are NOT talking of reducing exposure times by factor 13.5! See TRACE/CDS comparison.

The EUS Instrument

Environment

1. Thermal Loads

149 day cycle = 2,142 to 34,275 W/m2 (0.8 to 0.2 AU).

Need to address thermal balance for high load values and for variation of thermal input.

We must validate the designs through extensive modelling. Can we define test activities and facilities which could be used for such testing? What about optical degradation?

The EUS Instrument

Environment

2. Particle Environment at 0.2 AU

Cosmic Rays:- Non-solar cosmic rays about the same as for SOHO, or less.

Solar Wind:- Projecting naively from 1 AU values (~10 p/m3) we might expect 250 p/m3 in ‘normal’ conditions at 0.2 AU, with v ~ 400 km/s. Thus, we expect 106 hits/cm2.s (25x SOHO flux). Is this a worry? Perhaps not so much if the detectors are ‘buried’ (don’t view space directly) and if the protons are low enough energy (will be plenty of 100 keV protons, for example). (Note: 109 direct proton hits ‘will kill a CCD’ - not so an APS…)

Neutrons:- We might expect to see some. 15 min half life means that we may expect them - possibly only from flares but more often than for 1 AU. Concern over their cross section at the silicon lattice relative to protons. Needs investigation.

The EUS Instrument

Environment

2. Particle Environment at 0.2 AU (continued…)

Flares and shock (CME) particles:- Dose difficult to predict. Could argue that the chance of being hit by a flare proton(/neutron) ‘beam’ is the same as for, e.g. SOHO. What about from larger shocks? Would suggest that there is a greater chance of seeing energetic particles, but hard to calculate.

Note: Hadrons can cause damage to the silicon lattice which causes traps that can ‘steal’ charge which can be transferred to other parts of the image. The APS minimises the problem by not transferring charge.

Note: What about particle effects on optical surfaces? See CDS proton-gold coating study (subsurface bubbling).

The EUS Instrument

Detectors

Array Size I deal: 4kx4k

Restricting: 4kx2kLimit: 2kx2kNote: Spatial direction (along slit) 34 arcmin at0.5 arcsec pixel; Spectral direction 0.01-0.02Å/ pixel over about 50 Å. I f FOV reduced, we loseability to view full Sun at aphelion or completeactive region at perihelion. I f wavelength bandreduced to less than a ~40 Å, we would haveproblems obtaining temperature range required.

Number of detectors Maximum: 3Workable: 2Limit: 1Note: One f or each wavelength band. Only oneband would be VERY restricting. 2 bands would beOK but the call f rom the community has been f or3. We could use diff erent orders but 2 detectorsappears to be the only workable approach.

Minimum Exposure times I deal: 0.1 sTarget: 1sUnacceptable: >1 sNote: Should be selectable in range betweenminimum and maximum (see below).

Maximum Exposure times I deal: 100 s

The EUS Instrument

Detectors

Wavelength of Operation I deal: Sensitive to range 170-1200 Å

Workable: Sensitive to range 170-650 ÅLimit of acceptability: Sensitive toeither 170-220 or 580-630 Å bands.Note: EUV Prime Bands are: 170-220 Å, 580-630Å, and above 912 Å.

Dynamic Range I deal: 0 to 4096 counts (14 useful bits)Limit: 0 to 2048 counts (as long as wecan cope with any saturation with someevents)Note: This includes quiet Sun to fl ares. For theformer, we may expect counts of tens per pixelper exposure, f or the latter, counts of hundredsto even thousands per exposure.

Read out time I deal: 1 sWorkable Limit: 2 sNote: I mages are made by rastering. For a 10location raster of exposure 1 s and read out 1 s,we have total cadence of 20 s. The Sun is highlydynamic; the cadence must be as low as possible.

Pixel Size I deal: 5 micronWorkable/ limit: 9 micronNote: Smaller pixel size reduces size of theinstrument. The 9 micron pixel option produced a2.3m long instrument – probably too large.

The EUS Instrument

Detectors

Thermal Environment Variable (150 day) solar ‘constant’ in range 2000

to 34000 W/ m2 on f ront of instrument withthermal control maintaining detector temperature(cold finger to radiator, local heaters etc…).

Particle Environment Solar Background Protons: Factor of 25 increasein background protons (9 cm-3 at average of 300km/ s and 4x104K (3.5 eV)) at 1 AU gives 225 cm-3

at 0.2 AU.Solar Events: I ncreased chance of ‘storms’ f romsolar events due to vicinity, with increased dose(25 times) – anticipate storms with thousands ofhits per cm-2.Solar Neutrons: Neutron half lif e of 15.5 minmeans that only flare neutrons seen at 1 AU.Neutron flux is anticipated to be of order a f ew100 cm-3 at 0.2 AU.Cosmic Rays: Anticipate up to 30 particle hits ofabout 1 GeV protons/ cm2s (same as at L1).

Mass Baseline: 2.5 kgWorkable: 3.5 kgNote: 2.5 kg was estimated f or detector headplus electronics in original proposal. Mass isseverely restricted f or Orbiter.

The EUS Instrument

The consortium

Solar Orbiter

Rutherford Appleton Laboratory, UK

Mullard Space Science Laboratory, UK

Birmingham University, UK

Max Planck, Lindau, Germany

Padua University, Italy

Goddard Space Flight Center, USA

Oslo University, Norway

IAS, Orsay, France

NRL, Washington, USA

Documents

Solar Orbiter: Status and Schedule Richard Harrison, Rutherford Appleton Laboratory A high-resolution mission to the Sun and inner heliosphere